Atomic-force microscopy has become widely used to image surfaces of samples on a microscopic scale. Its popularity to a large extent is due to the fact that an atomic-force microscope (AFM) measures the force or force gradient between a sharp tip disposed on a cantilever and a sample surface at a picoNewton (pN) level as opposed to the tunneling current measured with a scanning tunneling microscope (STM). This of course allows the AFM to image insulating as well as conducting samples.
AFMs can be operated in either contact or intermittent contact modes. When operating in a contact mode, the deflection of a weak cantilever is kept constant while servoing the vertical extension of a piezoelectric scanner supporting the sample being imaged. The piezoelectric scanner is also rastered in an x-y plane to scan the surface of the sample. A map of the vertical extension of the piezoelectric scanner at various x,y coordinates of the sample surface, which is assumed to be proportional to a change in voltage on the piezoelectric scanner, reflects the topography of the sample surface. Unfortunately, problems exist in that soft samples are often damaged by the plowing action of the tip on the sample as the sample is rastered by the piezoelectric scanner in the x-y plane.
When operating in an intermittent contact mode, the base of a stiff cantilever is driven by a piezoelectric element which induces an oscillation at the free end of the cantilever. By driving the cantilever near its resonant frequency, an oscillation amplitude ranging from 20 to 100 nm at the free end of the cantilever can be achieved. This amplitude range is sufficient to inhibit the tip from sticking to the sample surface during each contact. To generate the image, the vertical extension of the piezoelectric scanner is servoed to maintain a constant drop in the oscillation amplitude. The piezoelectric scanner is also rastered in an x-y plane to scan the surface of the sample. In order to achieve high sensitivity, a high quality factor (Q) is necessary. Tapping mode cantilevers typically have Q values ranging from 100 to 1000 in air.
To enhance the measurement of force displacement curves, a modified form of atomic-force microscopy, referred to as interfacial force microscopy, has been developed. Interfacial force microscopes (IFMs) replace the cantilever with a differential-capacitance displacement sensor. Feedback is used to servo the net electrostatic torque of the sensor such that it cancels the torque resulting from tip-sample forces. As a consequence, the tip support remains at its rest position throughout the force profile. This feature eliminates the snap-to-contact instability which plagues weak cantilevers in the attractive force regime and correlates the tip-sample deformation directly to the vertical extension of the piezoelectric scanner in nanoindentation studies.
Feedback attempts to inhibit the common plate of the displacement sensor from actually deflecting which leads to rapid restabilization of the displacement sensor after hard collisions with pronounced surface features. However, this places considerable demands on the rate at which force signals drift since it is often necessary to image a large field of view at a slow lateral scan. Generally, a contact force in the range of 200 nN corresponds to a force signal in the order of 10 mV. As will be appreciated, the force signal has very little room to drift over the scan duration. A slow drift in the attractive force direction results in a slight increase in the contact force applied to the sample over the scan. Drifts in the repulsive force direction can pull the piezoelectric scanner completely out of feedback. Accordingly, improved imaging techniques are desired.
It is therefore an object of the present invention to provide a novel method and apparatus for intermittent imaging.